highlight server duration on trace summary page

It is helpful to tell at a glance how network latency affected a trace.
This change shows a highlight at the bottom of each span that contains
both client and server timings. The highlight shows what duration of the
span was spent in the server.

I arrived on this highlight style after a few other attempts. It keeps the
span text readable and uses the color for the next depth (which
hopefully makes it more intuitive).

Related to issue #196, but probably not everything envisioned.

![example](http://f.cl.ly/items/1L3R0F2z0m2o3a1H0w1r/zipkin-server-highlight-2.png)